1. Field of the Invention
This invention relates to a radiation image information read out system for scanning a stimulable phosphor plate with a light beam of stimulating rays to cause the stimulable phosphor plate carrying radiation image information to emit light according to the radiation image information stored therein in the form of radiation energy, and more particularly to a method of gradation processing for the radiation image information read out system in which the gradation of the finally obtained image is controlled by controlling the output level and the gain of the read out system at the time of image information read out step.
2. Description of the Prior Art
A stimulable phosphor stores a part of the energy of a radiation when exposed to the radiation like X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays and ultraviolet rays. Then, when the stimulable phosphor which has been exposed to the radiation is exposed to stimulating rays, light is emitted from the stimulable phosphor upon stimulation thereof according to the stored energy of the radiation.
As disclosed in U.S. Pat. No. 3,859,527, it has been known to use the stimulable phosphor for recording a radiation image. In more detail, the stimulable phosphor is first exposed to X-rays to memorize a radiation image therein, the stimulable phosphor is then scanned with stimulating rays to emit light according to the memorized image, the light emitted from the stimulable phosphor upon stimulation thereof is detected by a photodetector to obtain an image signal, and the image signal is used for controlling a laser beam for recording an image on a photosensitive film.
By use of this system, it is possible to obtain a radiation image useful for diagnosis. In order to obtain an image useful for diagnosis, the image should be reproduced into a desirable range of density. However, the image used for diagnosis has a very wide range of density in some cases like a mammograph or image of the head, and a comparatively narrow range of density in other cased like a plain image of the abdomen. Further, in case that a contrast medium or a lead protector is used, the image includes a portion of very low density and accordingly of very low level of the image signal. It is very difficult to record the image having such various types of density distribution in a proper density. Therefore, a proper gradation processing has relied upon the skill of the operator.
Further, the operator reads out the recorded image information and records the read out information once in a magnetic tape and determines the gradation processing condition by analyzing the image characteristics by use of a histogram and the like. In this case, a recording means having a large capacitor is necessitated and the structure of the recording apparatus becomes complicated and further it takes a long time for reproduction of the image.